Note: Descriptions are shown in the official language in which they were submitted.
107Z980
The present invention relates to a process for recovering unreacted
materials in the effluent from a synthesis of urea from carbon dioxide and
ammonia and, more particularly, it relates to an improved process for recov-
ering unreacted materials from a urea synthesis effluent while reducing the
amount of moisture entrained in the unreacted ammonia and carbon dioxide
separated from the urea synthesis effluent.
In the production of urea~ a total solution recycle process is well
known which comprises reacting carbon dioxide with ammonia under high temp-
erature and high pressure conditions~ conventionally known and recognized by
those skilled in the art as urea synthesis temperature and pressure conditions~
subjecting the resulting urea synthesis effluent to a plurality of stripping
or distillation stages under respective stepwise reduced pressures to separate
unreacted materials in the form of a gaseous mixture of ammonia, carbon
dioxide and water vapor in each of the respective stages, absorbing the gas-
eous mixture of unreacted materials discharged from the low pressure dis-
tillation stage in an absorbent~ increasing the pressure of the resulting
absorbate stepwise for use as an absorbent for the mixed gas separated in a
higher pressure decomposition stage, and recycling to the urea synthesis
z~ne the absorbate discharged from the final highest pressure decomposition
stage The urea synthesis reaction~ i.e.~ the reaction of ammonia with carbon
dioxide to form urea and water~ is a reversible reaction, so that the yield
of urea decreases with an increase of water content in the urea synthesis
reaction system. In order to improve the yield of urea~ it is necessary to
reduce to as small a value as possible the water content in the absorbate
recycled to the urea synthesis zone. For this purpose~ the absorption should
; be conducted under high pressure in the respective absorption stages and with
the use of a minimum amount of absorbent. Further~ the amount of water which
is evaporated and entrained in the gaseous mixture of unreacted materials
separated in the respective absorption stages should preferably be minimized
- 1 -
' ~
107Z980
for preventing the absorbate from being diluted. The gaseous mixtures separ-
ated in the separation zones under different pressure conditions have different
water contents. Of these, the gaseous mixture which is separated in the low
pressure separation zone operated under a gauge pressure of 0 - 5 kg/cm has
the greatest content of water. Especially when the unreacted materials in the
low pressure sta~e are stripped off with carbon dioxide fed into the bottom of
the separation zone of the low pressure decomposition stage in order to~com-
pletely separate the unreacted materials from the urea synthesis effluent,
the water or moisture content in the separated gaseous mixture disadvantage-
ously increases by an amount of water vaporized and entrained in the carbon
dioxide. Accordingly, an additional means is required to suppress this
increase in water content.
In order to overcome the above disadvantages, there has been
proposed in U. S. Patent No. 3,725,210 (issued April 3, 1973 to Mitsui Toatsu
Chemicals, Inc.) a process wherein the temperature at the top of the low
pressure rectification zone having a gauge pressure of 0 - 5 kg/cm is main-
tained at 60 - 120 C. and the temperature at the bottom thereof is kept at
100 - 140 C. The present invention contemplates providing an improvement in
the above process. In the known process, the temperature at the top of the
rectification zone is maintained in the range of 60 - 120 C. so that it is
possible to lower the vapor pressure and to reduce the water content in the
gaseous mixture. In order to maintain the top of the rectification zone at
such a low temperature, however, the gaseous mixture exhausted from the top
of the rectification zone must be fed to a reflux condenser wherein the
gaseous mixture is cooled to a temperature of 50 - 100C. to condense-the
water vapor therein, the condensed water being recycled to the top of the
rectification zone. Thus, the heat energy removed upon the reflux condensa-
tion results in a loss, increasing the quantity of heat required for heating
the separation zone by an amount corresponding to said heat loss.
- 2 -
107Z980
This invention seeks to provide a process for efficiently and
economically recovering unreacted ammonia and carbon dioxide from a urea
synthesis effluent.
This invention also seeks to provide a process for preparing
urea in high yield while recovering an unreacted mixed gas containing a
reduced amount of water from the urea synthesis effluent.
This invention also seeks to provide a process for effective-
ly recovering heat from a urea synthesis effluent for use in separation and
reoovery of the unreacted materials.
In developing this invention, the present inventor has made
intensive studies on these problems and found that the water content in the
gaseous mixture of unreacted materials separated in the low pressure separ-
ation stage can be reduced to a certain extent effectively by using the heat
contained in the ~rea synthesis effluent.
Thus this invention provides an improvement in the total
solution recycle urea synthesis process which includes the steps of reacting
in a urea synthesis zone carbon dioxide with ammonia under urea synthesis
temperature and pressure conditions, i.e., at a temperature of about 180 -
210 C. and under a pressure of about 200 - 260 kg/cm (gauge), to obtain a
~0 urea synthesis effluent containing urea, an excess of ammonia, unreacted
ammonium carbamate and water, subjecting said urea synthesis effluent to at
least two ammonium carbamate decomposition stages including at least one high
pressure stage at a gauge pressure above 10 kg/cm2 and a low pressure stage
at a gauge pressure below S kg/cm to separate from said urea synthesis
effluent gaseous mixtures each composed of ammonia, carbon dioxide and water
vapor in the respective separation zones of said high pressure and low
pressure decomposition stages, at least a portion of the separation zone of
the low pressure stage being a rectification zone having a top and a bottom,
contacting each of said gaseous mixtures with an absorbent in absorption
-- 3 --
.: . . . . . .
107Z980
~.ones each of which has substantially the same pressure as that of each of
the corresponding separation zones to absorb said gaseous mixtures in said
absorbent successively, and recycling the thus-obtained absorbate to the urea
synthesis zone, which improvement comprises cooling the urea synthesis
effluent from the separation zone of said high pressure stage to a temperature
of 105 - 170 C. by indirect heat exchange in a heat exchange zone with the
urea synthesis effluent existing in the bottom of the rectification zone of
said low pressure stage, reducing the pressure of said cooled urea synthesis
effluent to that of the low pressure stage, introducing the thus pressure
reduced effluent into the top of the rectification zone and at the same time
heating the bottom of said rectification zone by indirect heat exchange in said
heat exchange zone and in an additional heating zone to maintain the temper-
ature of the top of the rectification zone at 60 - 120 C. and the temperature ~-
of the bottom of the rectification zone at 100 - 140 C.
When the urea synthesis effluent from the at least one high
pressure decomposition stage operated under a gauge pressure of 10 - 170
kg/cm and at a temperature o~ 140 - 200 C. is reduced to a gauge pressure
of 0 - 5 kg/cm for flashing, the temperature thereof is reduced to 90 -
150 C. In this connection, however, when the urea synthesis effluent from the
high pressure separation stage is first subjected to an indirect heat exchange
with the urea synthesis effluent existing in the bottom of the rectification
zone of the low pressure separation stage so as to cool said urea synthesis
effluent from the high pressure stage of 100 - 170C. and is then reduced to
a gauge pressure of 0 - 5 kg/cm for flashing~ its temperature is reduced to
60 - 120 C. The feed of such a low temperature solution to the top of the
low pressure rectification zone reduces the temperature at the top of said
zone without any loss of heat, thereby ensuring reduction of the water content
in the separated gaseous mixture of unreacted materials.
The excess of heat contained in the hot urea synthesis
.. . . . .
1072980
effluent discharged from the high pressure separation stage is used for
heating the bottom of the low pressure rectification zone by indirect heat
exchange, resulting in a reduction in the amount of steam required for heating
the bottom of the low pressure rectification zone. This is because the indi~e~c
rect heat exchange process enables the steam or moisture content in the
gaseous misture separated in the low pressure separation stage to be reduced
much more than in the case where the urea synthesis effluent from the high
pressure separation zone is directly flashed into the low pressure separation
zone. Thus, the amount of latent heat of vaporization required for said
reduced amount of moisture is correspondingly reduced, leading to a reduction
in the amount of steam necessary for heating the bottom of the low pressure
rectification zone to a predetermined temperature.
In order completely to separate unreacted ammonia and carbon dioxide
from the urea synthesis effluent in the low pressure separation zone~ it is
preferred that the separation zone of the low pressure stage be divided into
two zones, i.e., a rectification zone and a stripping zone, and that the
urea synthesis effluent from the rectification zone is further fed into the
stripping zone wherein a stripping gas, e.g., carbon dioxide, is introduced
into the bottom thereof *o sbrip from the urea synthesis effluent the
residual unreacted materials for feed to the bottom of the rectification
zone. In this case, the absolute amount of moisture in the gaseous mixture
obtained from the top of the rectification zone is increased by the amount ofthat
entrained in the stripping gas. In order to reduce the amount of the
entrained moisture, thb temperature at the top of the rectification zone
should be maintained as low as possible. In practice of the present inven-
tion, the temperature at the top of the rectification zone is maintained in
the range of 60 - 120 C., within which range the temperature is varied
depending on the pressure, with the result that the moisture or water content -~
in said mixed gas can be suppressed and kept low even though the unreacted
_ 5 _
.~ ~ , . . , . : , , , :
1072980
ammonia and carbon dioxide are stripped off by use of the stripping gas such
as carbon dioxide. Thus, the total amount of water vapor evaporated and
entrained in said mixed gas does not increase considerably in spite of the
increased amount of gaseous mixture.
The stripping gas useful in feeding into the bottom of the expell-
ing or stripping zone to separatedthe unreacted ammonia and carbon dioxide
from the urea synthesis effluent is most preferably carbon dioxide due to
its high stripping efficiency. That is, carbon dioxide is sufficient when
usedin only a small amount to strip the unreacted materials, so that the
mixed gas discharged from the low pressure separation zone and composed of
the carbon dioxide and unreacted materials is correspondingly reduced in
amount, with a reduced total amount of entrained water vapor. The carbon
dioxide fed into the stripping zone is recovered in the low pressure
absorption zone by being absorbed in an absorbent together with the unreacted
ammonia and carbon dioxide separated in the rectification zone and the strip-
ping zone. Upon this absorption, the added or fed carbon dioxide serves to
reduce the equilibrium pressure of the low pressure absorption zone and
this, in turn, makes it possible to reduce the amount of absorbent used in
the low pressure absorption zone.
Although carbon dioxide is the preferred stripping gas, other
gases which may be used include hydrogen and inert gases such as nitrogen.
When~lthe preferred carbon dioxide is used as the stripping gas, a
portion of the starting carbon dioxide is generally used as the stripping
carbon dioxide fed into said.lstripping in an amount of 0.01 - 0.2 mol, pre-
ferably 0.02 - 0.1 mol, per mol of urea contained ihethe urea synthesis
effluent fed to the ~ripping zone. When the amount of carbon dioxide is
above 0.2 mol, the amount of distilled water is increased. On the other hand,
when the amount is below 0.01 mol, the unreacted materials tend not to be
stripped completely.
107Z980
In the practice of the present invention, the separation zone of
the low pressure stage is composed, as de~cribed hereinbefore, of a recti-
fication zone, a heat exchange zone, an additional heating zone and, if
desired, a stripping zone. These zones may be either integrally or separ-
ately constituted. When the stripping zone is used, the rectification zone
and the stripping zone are preferably integrally constituted, i.e., the
rectification zone in the upper half portion of the separation zone and the
expelling or stripping zone is in the lower half portion thereof.
The rectification zone may be constructed of bubble cap plate
column or other plate columns having functions corresponding thereto such
as sieve plate column, or may be constructed of a packed column having
functions similar to those of the above plate columns.
The stripping zone is generally constructed of a packed bed, and
the additional heating zone is composed of a~multitubular heat exchanger,
i.e., a heater of the one pass type, the reboiler type of the falling film
type, which is heated with steam or other hot medium.
The invention will now be described in more detail by way of
reference to the drawings in which: -
Figure 1 is a flowlchart showing one embodiment of the present
invention; and
Figure 2 is a flow chart showing another embodiment of the inven-
tion using a stripping treatment with carbon dioxide.
Referring to Figure 1, a urea synthesis effluent from which the
major proportion of unreacted ammonia and carbon dioxide have been separated
in a high pressure separation s~age (not shown) and discharged from the bottom
of the high ~r~ssure separation stage or a high pressure distillation towe~
(not shown) and which has a gauge pressure of 10 - 170 kg/cm and a temperature
of 140 - 200 C. is fed through line 10 to heat exchanger 12. In heat
exchanger 12, the urea synthesis effluent is heat-exchanged with a urea
107;Z980
synthesis effluent from the bottom of rectification æone 22 and is cooled to
105 - 170 C. preferably tool20 - 150 C. The thus cooled urea synthesis
efM uent is fed through line 14tthrough reducing valve 16 wherein its gauge
pressure is reduced to 0 - 5 kg/cm2~ preferably 1.5 - 3.0 kg/cm , and is
further fed through line 18 to the top of low pressure distillation tower 20
for flashing. By adiabatic expansion, the urea synthesis effluent fed to
the top of rectification zone 22 is cooled to 60 - 120 C. The urea
synthesis effluent, from which a gaseous mixture of unreacted ammonia,
carbon dioxide and water vapor has been separated at the top of rectification
zone 22, flows down through the rectification zone to the bottom thereof.
Part of the urea synthesis effluent is withdrawn from the bottom of rectifi-
cation zone 22 through line 24 and fed to heat exchanger 12 to be heated by
heat exchange with the urea synthesis effluent from the high pressure
separation zone. As a result, the heated urea synthesis effluent reache~
a temperature of 100 - 140 C., preferably 115 - 135 C., and is recycled to
the bottom of rectification zone 22 through line 26. Further, a portion of
the urea synthesis effluent withdrawn through line 24 is fed through line
28 to reboiler 30 wherein the same is likewise heated to 100 - 140 C.,
preferably llS - 135 C., by means of the steam fed through line 32. The
thus heated urea synthesis effluent is also recycl~d toothe bottom of recti-
fication zone 22 through line 34 and line 26. The urea synthesis effluent
which is recycled from the bottom of rectification zone 22 through heat
exahanger 12 and reboiler 30 may be passed through the heat exchanger and
reboiler in parallel as shown in Fegure 1. Alternatively, the heat exchanger
and the reboiler may be arranged in series. When the urea synthesis
effluent from the bottom of rectification zone 22 is heated in heat exchanger
12 and reboiler 30, most of the unreacted ammonia and carbon dioxide remain-
ing in the urea synthesis effluent are separated in the form of a gaseous
mixture and recycled into the bottom of rectification zone 22 together with
- 8 -
.. . .
.~, ' ' . .
107Z980
the urea synthesis effluent. The gaseous mixture is separated therein from
the urea synthesis effluent and ascends through rectification zone 22 while
condensing a portion of water ~apor contained in the gaseous mixture. The
mixed gas with reduced water content is exhausted from the top of rectification
zone 22 through line 36 together with the gaseous mixture separated by said
flashing of the urea synthesis effluent, and fed to a low pressure absorption
zone, not shown.
When it is desired to strip the urea synthesis effluent in the
bottom of rectification zone 22 by means of carbon dioxide, the urea synthe-
sis effluent is further fed to stripping zone 38 composed of a packed column
which is provided in the lower portion of distillation tower 20 as shown in
Figure 2, The urea synthesis effluent descends through stripping zone 38
countercurrently to carbon dioxide which is fed through feed tube 40 pro-
vided at the bottom of the stripping zone thereby to separate substantially
all of the residual unreacted ammonia and carbon dioxide from the urea
synthesis effluent. The thus separated urea synthesis effluent is discharged
from the bottom of distillation tower 20 through line 42 to a subsequent
concentration step, not shown. The gaseous mixture composed of the carbon
dioxide fed to stripping zone 38 and the ammonia and carbon dioxide separated
therein is fed from stripping zone 38 to the bottom of rectification zone 22
for mixing therein with the gaseous mixture separated in reboiler 30 and
heat exchanger 12. The resulting gaseous mixture ascends through rectifi-
cation zone 22 and is exhausted from the top thereof through line 36.
According to the present invention, the urea synthesis effluent
from the high pressure separation stage is first subjected to heat exchange
with a urea synthesis effluent existing in the bottom of the rectification
zone of the low pressure separation zone to cool said effluent to a pre-
determined temperature and it is then reduced in pressure, so that the
temperature of the urea synthesis effluent is in turn lowered due to adia-
batic expansion in a greater degree than that attained by a mere adibatic
_ g _
- . .
.
~072980
expansion technique and, thus, the temperature at the top of the rectifica-
tion zone can be maintained in a suitable temperature range. In addition,
the heat removed by the heat exchange can be used as part of the heat source
for heating the bottom of the rectification zone. That is, the heat of the
urea synthesis effluent from the high pressure separation zone is caused to
be transferred from the top of the low temperature rectification ~one ~o the
bottom of the rectification zone with a high temperature by use of the
expansion of the high pressure urea synthesis effluent. Thus, the heat of
the hot urea synthesis effluent can be effectively used as a heat source for
the rectification. This results in reduction in the amount of steam required
for rectification of the urea synthesis effluent in the low pressure decom-
position stage. Further, neither a condenser for condensing the water vapor
in the gaseous mixture from the top of the rectification tower, nor cooling
water for the condenser is required. Additionally, the temperature at the
top of the rectification zone is maintained so low that the moisture content
in the gaseous mixture of the unreacted materials separated from the recti-
fication zone is reduced and the amount of water recycled to the urea
synthesis zone is also reduced, leading to a high yiald of urea in the urea
synthesis zone. I
The present invention will be particularly illustrated by way of
the following non-limitingexamples.
E~ample 1
As a control test, a urea synthesis efM uent withdrawn from the
` bottom of a high pressure distillation tower operated under a gauge pressure
of 19 kg/cm at a temperature at the bottom of 180C. and composed, in kg/hr,
of 1130 of urea, 107 of ammonia, 28 of carbon dioxide and 447 of water was
flashed, as such, under adiabatic conditions into the top of the low pressure
distillation tower operated under a gauge pressure of 2.7 kg/cm in accordance
with a known process. As a result, a gaseous mixture composed, in kg/hr, of
, .
-- 10 --
,
107Z980
73.8 of ammonia, 25.4 of carbon dioxide and 53.7 of water was separated from
the urea synthesis effluent, and the temperature of the effluent was
lowered to 120 C. The urea synthesis effluent obtained after said flashing
had a composition composed, in kg/hr, of 1130 of urea, 33.2 of ammonia, 2.6
of carbon dioxide and 393.3 of water. Thereafter, the urea synt~qesis
effluent was further distilled in a distillation tower containing 6 plates.
The resulting solution from the bottom of the tower was heated to 130 C. by
means of a reboiler. The urea synthesis effluent discharged from the bottom
of the distillation tower contained 2.0 kg/hr of residual ammonia and 1.5
kg/hr of residual carbon dioxide. The mixed gas exhausted from the top of
the distillation tower, i.e., a combination of the flashed gas and the gas
separated by distillation, had a composition, in kg/hr, of 105 of ammonia,
26.5 of carbon dioxide and 76.4 of water. In the reboiler~ 88 kg/hr of
5 kg/cm (gauge) steam was consumed.
Then, to show the advantages of the present invention, a urea
synthesis effluent discharged from the bottom of the high pressure distill-
ation tower was treated without reducing its pressure in accordance with the
process of the invention. That is, said urea synthesis effluent was passed
through the tubes of a multi tubular heat exchanger for heat exchange with a
urea synthesis effluent from the bottom of the low pressure distillation
tower passed through the shell of said heat exchanger. As a result, the
high pressure urea synthesis effluent fed from the heat exchanger was lowered
in temperature to 150 C. The urea synthesis effluent was flashed into the
top of a low pressure distillation tower operated under a gauge pressure of
2.7 kg/cm thereby to separate therefrom a mixed gas composed, in kg/hr, of
56.3 of ammonia, 16.5 of carbon dioxide and 28.3 of water and to lower the
effluent temperature to 110C. The urea synthesis effluent obtained after
the flashing contained, in kg/hr, 1130 of urea, 50.7 of ammonia, 11 5 of
carbon dioxide and 418.7 of water. The amount of water evaporated upon
.
.. . , . :, .. . .
~07;Z980
flashing was reduced to almost half of that attained by the known process by
lowering the temperature of the urea synthesis effluent by 30 C. prior to
the pressure reduction by the heat exchange. The solution obtained after
flashing was distilled in a distillation tower and the resulting solution at
the bottom of the tower was heated by means of the aforementioned reboiler
and said heat exchanger and maintained at 130C. As a result, only 2.3 kg/hr
of ammonia and 1.7 kg/hr of carbon dioxide remained in the urea synthesis
effluent discharged from the bottom of the distillation tower, while the
mixed gas exhausted from the top of the distillation tower contained, in
kg/hr, 104.7 of ammonia, 26.3 of carbon dioxide and 52.8 of water. Thus,
even though the amounts of ammonia and carbon dioxide remaining in said urea
synthesis effluent discharged from the bottom of the distillation tower were
substantially the same as those in the case of the known process, the total
amount of evaporated water was reduced to about 70% of that in the known
process. In the process of the invention, only 64 kg/hr of 5 kg/cm2 (gauge)
steam was consumed in the reboiler, the steam being reduced by 24 kg/hr
when compared with the known process. In other wQrds, the amount of heat
required in the reboiler was reduced by an amount corresponding to the
difference in amount of evaporated water. Additionally, there was a reduced
water content in the absorbate which absorbed the separated gaseous mixture
and was recycled to the urea synthesis column, so that the yield of urea was
improved.
Example 2
The urea synthesis effluent obtained from the distillation tower
in Example 1 was further treated in a stripping zone provided beneath the
distillation tower in direct connection therewith and composed of a 5 m.
high packed bed. Into the bottom of the stripping zone was blown 33 kg/hr
of carbon dioxide for stripping. me resulting mixed gas ascended through
the stripping zone and also the rectification zone, and was exhausted from
- 12 -
-
1072980
the top of the distillation tower together with the flashed gas and the gas
separated by the distillation.
When the known process as described in Example 1 without using the
heat exchanger of the present invention was combined with this stripping
process, the gas exhausted from the top of the distillation tower had a
composition, in kg/hr, of 106.0 of ammonia, 60.0 of carbon dioxide and
86.4 of water, and the urea synthesis effluent discharged from the bottom of
the packed col = contained ammonia and carbon dioxide each in an amount of
1.0 kg/hr.
On the other hand~ when the urea synthesis effluent was treated
by the process of the present invention combined with said stripping process,
the composition of the gas from the top of the distillation tower was composed,
in kg/hr, of 105.8 of ammonia, 59.8 of carbon dioxide, and 59.3 of water.
The temperature at the bottom of the packed col = was 125 G and the urea
synthesis effluent discharged from the bottom of the packed col = contained
ammonia and carbon dioxide each in am amount of 1.2 kg/hr. Similarly as in
Example 1, the amount of evaporated water was reduced to about 70% of that in
the case of the known process.
- ~ :
, . .
